This invention relates to an improved multi-layered (composite) card and to apparatus and methods for the manufacture of composite (multi-layered) cards.
Composite cards are required in many applications where greater card strength and extended durability of the cards are needed or desired; e.g., it is desired to have a card which will typically last in excess of five years rather than the typical two years. Some of these applications include, for example, national identification (ID) cards, college ID cards, smart cards, driver""s licenses, cards for holding permanent and long lasting records such as medical records, and security cards. These composite (multi-layered) cards are required to last for a long time even where high temperature levels (hot and cold) and/or a high degree of humidity may be encountered
To better understand the discussion to follow, reference is first made to FIG. 1A, which shows a typical prior art composite card formed of a number of layers of PVC (polyvinyl chloride) and PET (polyester terepthalate). The prior art card of FIG. 1A, includes: (a) a core PVC layer, 10, Which is approximately 0.024 to 0.026 inches thick; (b) a clear laminate (12a, 12b) layer of PET material on either side of the core layer 10; each PET layer being approximately 0.002 inches thick , with one PET layer 12a being attached to the top surface (101a) and one PET layer 12b being attached to the bottom surface (101b) of core layer 10; and (c) a third PVC layer (14a, 14b), which may be 0.001 to 0.002 inches thick, may be attached to each one of the outer surfaces (121a, 121b) of each one of the PET layers 12a, 12b. 
The PVC layer is generally used in greater quantity because it is inexpensive and because it is easy to personalize (i.e., information can be written into the PVC layer with relative ease). The PET layer is generally used because it adds strength to the card, is heat resistant, and is relatively impervious to humidity. The PET layers 12a, 12b are normally bonded (attached) to the core PVC layer by techniques such as heat lamination (e.g., at 275xc2x0 F.). In general, composite cards may be manufactured by bonding two, or more, different layers together. The bonding process may include laminating, heat gluing and/or any other suitable bonding technique.
Fixed information is normally applied to the outer surface (101a, 101b) of core layer 10 by offset and/or screen printing. Variable information is normally applied to the outside surfaces 121a, 121b of layers 12a, 12b, or to surfaces of layers 14a, 14b by embossing or thermal printing. Note that the core 10 may be formed of a split core comprised of two(2) layers (10a, 10b) of PVC material, with each layer having a thickness of approximately 0.013 inches, as shown in FIG. 1B.
Cards may also be formed as shown in FIG. 1C. In this example, a core of white PET material is covered on top and bottom by an adhesive coated PVC layer.
A significant problem with prior art cards is that the PET layers and the PVC layers have different coefficients of expansion (and contraction) as a function of temperature and stress. As a result, there are significant problems at the interface between the PET and PVC layers. The stress at the interface causes a weakening of the bond between the PET and the PVC layers resulting in distortion (warping) of the cards and/or the separation of the layers.
A finished card acts similarly to an I-Beam. Its strength comes from the outer layers which includes the layers bonded on the top and bottom of the core stock. Just like the strength of an I-Beam comes from its outside surfaces so does a card. This means that the thicker the outer layer of PET, the sturdier the card. However, the coefficient of expansion of the PET, as a function of temperature, is about one-half that of PVC. Thus, due to mechanical stresses at the PET/PVC interface, the bond between the two layers may be at or near the breaking point. Therefore, direct bonds of PVC and PET layers formed by heat are highly stressed and subject to warping and/or subsequent separation.
Also, known prior art composite cards with a white PET core, of the type shown in FIG. 1C, suffer from xe2x80x9cZ-axisxe2x80x9d failure, or weakness in the thickness direction resulting in the separation of the card layers. This problem with cards having white PET layers is that the whiteners (e.g., a white powder such as titanium dioxide) or other substances (e.g., barium sulfate) within the PET layer contain air bubbles which diffract light and act as opacifiers. However, due to the crystalline structure of PET the base surfaces of these whiteners (and other like substances) do not bond (i.e., do not xe2x80x9cwet outxe2x80x9d by) to the PET layer, causing the PET film to act as a weak foam.
Regarding the other components used to make cards, typically, most commercial lots of PVC have inconsistent surface contamination, mainly processing waxes, resulting in quality testing inaccuracies and quality assurance problems during card manufacture.
Conventional adhesives used to glue the PVC and PET layers together have not been satisfactory because the adhesives tend to be moisture sensitive. As a result, under extended heat or humidity conditions the adhesive bond is weakened and the card layers separate.
It is therefore desirable to have a composite card in which the problems discussed above are eliminated and the card has a structure which, under normal wear and tear, is durable for an extended period of time (e.g., in excess of 5 years) and is relatively cheap to manufacture.
Cards embodying the invention include a buffer layer between a PET layer and a PVC layer. The buffer layer is selected to have temperature and stress responsive properties intermediate those of the PET and PVC layers. Thus each buffer layer provides improved bonding to a PET layer on one side and to a PVC layer on the other side. By way of example, where the temperature coefficient of PET is in the range of 25xc3x9710xe2x88x926 cm per cm for each degree Centigrade (i.e., a one centimeter sample, when heated, will expand 0.000025 centimeters for every xc2x0 C. of heat rise) and the temperature coefficient of PVC is in the range of 50xc3x9710xe2x88x926 cm per cm for each xc2x0 C., then the buffer layer will be made to have a temperature coefficient somewhere between. 25 and 50xc3x9710xe2x88x926 cm per cm xc2x0 C. (e.g., in the range of 37.5xc3x9710xe2x88x926 cm per cm xc2x0C.). That is, where one centimeter of PET and one centimeter of PVC, when heated, expand by 0.000025 centimeters and 0.000050 centimeters, respectively, a one centimeter sample of the buffer layer will expand by an amount which lies between the change of the PET and the PVC. In addition to the buffer layer having a specified temperature coefficient, it is preferred to be relatively impervious to humidity; i.e., it has a low degree of moisture absorbance. As a result, a buffer layer protects the surface area of an associated PVC layer from contact with and from absorbing humidity in the surrounding atmosphere. The buffer layer is also selected to be elastic and pliable so as not to be subject to xe2x80x9ccrackingxe2x80x9d when its associated PET and or PVC layers are subjected to bending and like stresses. The buffer layer must also bond easily to PET and PVC. It is also desirable that the cost of the buffer layer be less than the cost of the PET.
Applicant discovered that materials suitable for forming a buffer layer having the properties discussed above include a copolymer of PE (polyethelene) and PVC and thin films of PVC of low molecular weight. Applicant also discovered that a copolymer of PVC and polyvinyl acetate with 10% to 30% acetate content produces a material suitable for forming a desirable buffer layer. Other suitable materials which may be used include: polyolefins, ethylene, vinyl acetate, amorphous polyesters and acrylics.
The buffer material used to practice the invention may be stretched during the course of being manufacture beyond its elastic limits in one or preferably two directions (e.g., the xe2x80x9cmachine directionxe2x80x9d which may be defined as the direction in which the buffer material is being drawn through a processing machine and the xe2x80x9ctransverse directionxe2x80x9d which is generally perpendicular to the machine direction) and heat annealed to prevent stress cracking.
The thickness of the buffer layer may vary over a wide range (e.g., from less than 0.001 inches to more than 0.01 inches). The limitation on the thickness of the buffer layer being determined by the maximum allowable thickness of the card (typically 0.03 inches) and the thickness of the core portion and outer layers (i.e., the PVC and PET layers).
Therefore, cards embodying the invention include a buffer layer between a PET layer and a PVC layer, with the buffer material being made up of a material to reduce or mitigate stress at its interface with a layer of PET on one side and a layer of PVC on the other side. The buffer layer reduces the bonding problem. The buffer layer combines the properties of PVC material and the PET material, thus reducing the stress at all the interfaces.
The strength of cards embodying the invention is due to a great extent to the outer layers which make their construction sturdier. An outer layer may incorporate a PET layer, a buffer layer and a PVC layer. The thickness of the various layers are adjusted to maintain a typical overall card thickness of 0.030 inches.
Using a buffer layer also enables good products to be formed from a wider range of PVC suppliers because it allows a higher PVC defect rate and wider range of PVC product properties to be used more successfully.